1. Field of the Invention
One disclosed aspect of the embodiments relates to an imaging apparatus and an imaging method, and in particular, relates to an imaging apparatus and an imaging method for performing automatic focus adjustment based on an image obtained by imaging.
2. Description of the Related Art
Conventionally, an autofocus device in a video camera or a digital still camera uses an image sensor to generate a focus evaluation value, which indicates the sharpness of the screen, from an image signal obtained by photoelectrically converting an object image. A mainstream method is a method of adjusting the focus by controlling the focus lens position to maximize the focus evaluation value.
In a case of capturing an image of a person, however, if the background has a higher contrast than that of the person, which is the main object, the person may be out of focus, and the background may be in focus.
To solve such a problem, an imaging apparatus including a face detection function is known. For example, there are imaging apparatuses that can set a focus detection area including the area of a detected face and perform auto-focusing (AF).
Further, a conventional focus adjustment method cannot identify an object and therefore performs the following control to achieve a desirable focusing operation on many objects. Specifically, the conventional focus adjustment method performs control so that if the focus evaluation value is large, the responsiveness is reduced on the assumption that the object may be in focus, and so that if the focus evaluation value is small, the response speed is increased on the assumption that the object may be out of focus.
Thus, for example, an object having a low contrast, such as a human face, has a low focus evaluation value even if the object is in focus as illustrated in FIG. 8. Thus, the conventional focus adjustment method may perform control to increase the response speed even if the object is in focus. This may impair the stability.
Although such a problem existed in the past, an imaging apparatus that can detect a face particularly limits the focus detection area for generating the focus evaluation value to a detected face and sets the focus detection area not to include another object to prevent the object from dropping. This makes the above phenomenon noticeable.
To solve such a problem, the following method is discussed in Japanese Patent No. 5017195. According to Japanese Patent No. 5017195, a minute driving operation is used, which drives the focus lens back and forth by a predetermined amount to determine the driving direction. A mountain climbing operation is used, which drives the focus lens in the direction in which the focus evaluation value increases by a predetermined amount in the determined driving direction. The minute driving operation and the mountain climbing operation are used to achieve a focus adjustment operation for adjusting the focus.
In the focus adjustment operation, the amount of driving of the focus lens in each operation when a face has been detected is set to be smaller than that when a face has not been detected.
Further, if the focus evaluation value obtained by the minute driving is smaller than a threshold, the operation transitions to the mountain climbing operation.
Then, the threshold is set smaller when a face has been detected than that when a face has not been detected, thereby stabilizing the focus adjustment operation even in a case of an object having a low contrast, such as a human face.
The technique discussed in Japanese Patent No. 5017195, however, has the following problem. That is, in a human face, the main factors that affect the contrast, such as the contour, the color of the skin, the features, and the hairstyle, vary from person to person. Thus, even when a human face has been detected by applying the technique discussed in Japanese Patent No. 5017195, if the detected contrast is low, the operation may transition to the mountain climbing operation, which greatly changes the focus state. This may bring the background into focus.